Manufacture of varistors with a passivation layer

ABSTRACT

A method ( 1 ) of manufacturing an electronic component comprising an electro-ceramic body and conductive terminations is disclosed. The method ( 1 ) includes the steps of providing ( 10 ) an electro-ceramic body, applying ( 11 ) a termination material to the body, applying a passivation material, firing to cure the termination material to provide terminations and plating ( 15 ) the terminations. The component is fired ( 12 ) before application of the passivation material in a first stage to achieve a porous termination material of sufficient strength for subsequent processing. The passivation material is applied ( 13 ) to the porous passivation material and the body after said first stage firing. The component is subsequently fired ( 14 ) in a second stage after application of the passivation material, said second stage firing having parameters causing at least some of the passivation material overlying the terminations to diffuse into the porous termination material while leaving substantially intact the passivation material over the body. The termination material further comprises a sinter inhibitor (Pt, at 1.5 wt %) to assist with control of porosity of the termination material during first stage firing.

FIELD OF THE INVENTION

The invention relates to manufacture of voltage dependent non-linearresistors (“varistors”) or other electro-ceramic electronic componentshaving a partly-conducting body.

PRIOR ART DISCUSSION

Varistors have been manufactured for many years. They comprise anelectro-ceramic body, typically ZnO, and terminations for electricalcontact. Most varistors are for surface mounting, and so theterminations are on end faces and around extremities of the four sidefaces. Since the 1980's many varistors have had interleaved internalelectrodes. U.S. Pat. No. 6,535,105 (AVX), U.S. Pat. No. 5,565,838 (AVX)and U.S. Pat. No. 5,387,432 (Hubbell) describe such varistors.

A problem in manufacture of varistors is that of consistently achievingaccurate plating of the terminations. This problem has become more acutein recent years with increasing miniaturisation. U.S. Pat. No. 6,535,105describes application of a resin coating (“passivation”) which protectsthe ceramic from plating. However the resin coating underlies the metalof the terminations and may reduce quality of the electrical pathbetween the inner electrodes and the termination plating. U.S. Pat. No.5,565,83 describes an approach in which the terminations are sputteredover a passivation coating. Again, it appears that there may beinsufficient consistency in electrical contact with the internalelectrodes. U.S. Pat. No. 5,387,432 discloses passivation compositions.

The invention is directed towards providing a manufacturing method forvaristors, with improved consistency and accuracy in plating ofterminations.

SUMMARY OF THE INVENTION

According to the invention, there is provided a method of manufacturingan electro-ceramic component comprising an electro-ceramic body andconductive terminations, the method comprising the steps of providing anelectro-ceramic body, applying a termination material to the body,applying a passivation material, firing to cure the termination materialto provide terminations, and plating the terminations, characterized inthat,

-   -   the component is fired before application of the passivation        material in a first stage to achieve a porous termination        material of sufficient strength for subsequent processing,    -   the passivation material is applied to the porous passivation        material and the body after said first stage firing, and    -   the component is subsequently fired in a second stage after        application of the passivation material, said second stage        firing having parameters causing at least some of the        passivation material overlying the terminations to diffuse into        the porous termination material while leaving substantially        intact the passivation material over the body.

Because the passivation material diffuses into the termination materialduring second stage firing there is excellent plating of theterminations and yet still good protection for the electro-ceramic bodyduring the plating. This assists with improving product yield.

In one embodiment, the termination material comprises Ag, glass frit,and carrier.

In one embodiment, the termination material further comprises a sinterinhibitor to assist with control of porosity of the termination materialduring first stage firing.

In one embodiment, the sinter inhibitor has a melting point greater thanthat of a primary component of the termination material.

In one embodiment, the sinter inhibitor comprises Pt.

In one embodiment, the Pt is present in a concentration of 0.1 wt % to 4wt %.

In one embodiment, the Pt concentration is approximately 1.5 wt %.

In another embodiment, the sinter inhibitor comprises alumina.

In one embodiment, the passivation material comprises glass, binder, andwater.

In another embodiment, the passivation material is applied by spraying.

In one embodiment, the spraying is conducted in a heated air flow.

In one embodiment, the first stage firing plateau temperature is in therange of 420° C. to 510° C.

In one embodiment, the first stage firing plateau temperature is in therange of 480° C. to 490° C.

In one embodiment, the first stage firing duration is 15 mins to 40mins.

In a further embodiment, the first stage firing duration is 20 mins to30 mins.

In one embodiment, the second stage firing plateau temperature is in therange of 630° C. to 710° C.

In one embodiment, the second stage firing plateau temperature is 650°C. to 670° C.

In one embodiment, the second stage firing duration is in the range of 5mins to 35 mins.

In one embodiment, the second stage firing duration is in the range of 5mins to 15 mins.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the followingdescription of some embodiments thereof, given by way of example onlywith reference to the accompanying drawings in which:

FIG. 1 is a flow diagram illustrating a varistor manufacturing method ofthe invention;

FIGS. 2( a) to 2(c) are diagrams (not to scale) illustrating thevaristor at stages of: (a) termination paste applied and varistor firedin first stage, (b) passivation coating applied, and (c) after secondstage firing;

FIGS. 3( a) and 3(b) are profile plots showing firing temperatures vs.time for first and second stage firing respectively; and

FIGS. 4( a), 4(b) and 4(c) are cross-sectional images of (a) ceramicbody and paste after first firing stage, (b) after second firing stage,and (c) after Nickel (Ni) and Tin (Sn) plating.

DESCRIPTION OF THE EMBODIMENTS

Referring to FIG. 1 a method 1 for manufacturing a varistor is shown.

In a step 10 a ZnO ceramic body with internal electrodes is manufacturedin a conventional manufacturing technique. In a step 11 terminationpaste of Ag, Pt, glass frit, and vehicle is applied by dipping theterminations into the paste. The addition of Pt as a sinter inhibitor(in a concentration range of 0.1 wt % to 4.0 wt %) in the terminationmaterial is very advantageous as it helps to control the level ofporosity by means of controlling the time/temperature of the firingprocess. The inhibitor is platinum, such as that commercially known as“platinum black” with a specific surface area of 12-18 m²/g. Thetermination paste material comprises in this embodiment 74 wt % (Ag) and1.5 wt % (Pt), the remaining being glass frit and vehicle.

In a step 12, there is first stage firing to a plateau temperature inthe range of 420° C. to 510° C., as shown by the plot of FIG. 3( a).Also, FIG. 2( a) diagrammatically shows the terminations after the firststage firing. The component is indicated by the numeral 20, the ceramicbody (with internal electrodes) by 21, and the termination paste by 22.The temperature of the first stage firing is preferably within thesmaller range of 480° C. to 490° C. The first stage firing plateauduration is in the range of 15 mins to 40 mins and is preferably withinthe smaller range of 20 mins. to 30 mins.

The sinter inhibitor has a higher melting point than the maintermination paste component, Ag. It retards densification duringsintering. For a pure silver termination the interfacial energy betweenthe particles is relatively low and so at the first stage firingconditions a greater level of densification would be achieved. Howeverthe addition of the platinum in the silver termination materialincreases the interfacial energy required to initiate densification. Atthe given first firing conditions the higher interfacial energyrequirement leads to a low level of densification.

The aim of the first stage firing is to melt the glass in the pastefirst sufficiently to bind the Ag particles together without allowingthe silver to densify. The termination is thus porous, however, it hassufficient strength to allow subsequent processing. FIG. 4( a) is animage of a termination after the first stage firing in which theporosity of the paste is visible.

In a step 13 a passivation material of silica glass fit is applied byspraying the complete component whilst heating and rotating to ensurethat all sides are adequately coated. In this embodiment this isachieved by “fluidizing” the components in a warm airflow in anatmosphere of misted passivation material. The passivation materialsprayed on comprises 13.5 wt % glass frit, 1 wt % latex binder, and theremainder deionised water. The “fluidizer” operates with an outlettemperature of between 39° C. and 44° C. with an airflow of c. 100 m³/hrto ensure thorough drying of the components. FIG. 2( b) shows acomponent 25 after application of the passivation material, 26, clearlyextending over all surfaces.

In a step 14 the components are fired in a second stage with a profileshown in FIG. 3( b). The range of plateau temperature is 630° C. to 710°C. and most preferably in the sub-range of 650° C. to 670° C. Thecorresponding times are 5 mins to 35 mins and most preferably 5 mins to15 mins. This causes the passivation to melt and migrate into thetermination due to its porosity, leaving a hard conductive Agtermination 31 as illustrated in FIG. 2( c). This diagram shows that thepassivation layer 26 over the termination paste 22 has “disappeared”—diffusing into the porous termination during the second stage firing.However, the passivation layer 26 remains on the surfaces of the ceramicbody between the terminations, to protect the ceramic during subsequentprocessing. A cross-section of the termination material after secondstage firing is shown in FIG. 4( b) in which the reduced porosity isapparent.

In a step 15 a nickel barrier layer followed by a solderable Sn or Sn/Pballoy is selectively plated onto the termination by electroplating asshown in FIG. 4( c). Because the ceramic between the terminations isprotected by the passivation there is little risk of it being plated. Atthe same time the terminations are well plated because the passivationlayer has diffused into the terminations during second stagefiring—leaving exposed Ag as an excellent host for electroplating.

The passivation material may alternatively be applied by rotation in a“pan coater”, such that the units are tumbled in a heated chamber intowhich a fine spray of the passivation material is injected.

It will be appreciated that the invention provides for excellentselectivity in plating of terminations. Heretofore, the plating processparameters for such devices have required very tight control onvariations in electroplating chemistry, time and current density. Withthe invention, there is greatly reduced possibility of glass remainingon the surface of the termination, making the device more tolerant ofvariations in the plating process. Also, since there is diffusion of theglass into the termination it is possible to increase the amount ofglass deposited on the component. The thickness of glass on the ceramicsurface is proportional to the level of overplate that can occur. Priorart processes with tight control on the plating parameters would havetypically a 7% yield loss due to overplate with a typical passivationglass deposit of 2-3 um. This overplate yield has been substantiallyreduced to less than 1% with the invention. In the prior processeswhereby glass passivation is applied over the ceramic body and anormally fired termination (typically recommended firing temperature of600′C) there is a trade-off between the amount of glass, the level ofoverplate and the thickness of the plated metals on the termination. Iftoo much glass is applied there is a high risk that some of the glasswill remain on the termination surface thus making plating moredifficult and potentially resulting in a reduced metal coating. Thebenefit of a thicker glass laydown on the ceramic body is that therewill be a low risk of plated metals being deposited on the body.Similarly if too little glass is applied, while the plating of thetermination will be easier, there is the increased risk of platingoccurring on the ceramic body due to the thin layer of glasspassivation.

The invention overcomes these constraints because it has enabled theapplication of a thicker layer of glass passivation thereby reducing theoverplate yield loss. This improved process has enabled a glass laydownof ˜6 um with excellent plating on the terminations and an overplateyield loss of typically less than 1%.

The invention is not limited to the embodiments described but may bevaried in construction and detail. Other materials are suitable assinter inhibitors, and one based on an alumina material and having asurface area of 13.5 m² has shown a similar effect. Low porosity can beachieved by firing a silver-only termination material, however thisrequires a lower temperature and while the porosity can be achieved themechanical strength of the termination is lower and thus furtherprocessing of the devices is more difficult.

1. A method of manufacturing an electro-ceramic component comprising anelectro-ceramic body and conductive terminations, the method comprisingthe steps of providing (10) an electro-ceramic body, applying (11) atermination material to the body, applying a passivation material,firing to cure the termination material to provide terminations, andplating (15) the terminations, characterized in that, the component isfired (12) before application of the passivation material in a firststage to achieve a porous termination material of sufficient strengthfor subsequent processing, the passivation material is applied (13) tothe porous passivation material and the body after said first stagefiring, and the component is subsequently fired (14) in a second stageafter application of the passivation material, said second stage firinghaving parameters causing at least some of the passivation materialoverlying the terminations to diffuse into the porous terminationmaterial while leaving substantially intact the passivation materialover the body.
 2. A method as claimed in any preceding claim, whereinthe termination material comprises Ag, glass frit, and carrier.
 3. Amethod as claimed in claim 2, wherein the termination material furthercomprises a sinter inhibitor to assist with control of porosity of thetermination material during first stage firing.
 4. A method as claimedin claim 3, wherein the sinter inhibitor has a melting point greaterthan that of a primary component of the termination material.
 5. Amethod as claimed in claim 3 or 4, wherein the sinter inhibitorcomprises Pt.
 6. A method as claimed in claim 5, wherein the Pt ispresent in a concentration of 0.1 wt % to 4 wt %.
 7. A method as claimedin claim 6, wherein the Pt concentration is approximately 1.5 wt %.
 8. Amethod as claimed in claim 3 or 4, wherein the sinter inhibitorcomprises alumina.
 9. A method as claimed in any preceding claim,wherein the passivation material comprises glass, binder, and water. 10.A method as claimed in any preceding claim, wherein the passivationmaterial is applied by spraying.
 11. A method as claimed in claim 10,wherein the spraying is conducted in a heated air flow.
 12. A method asclaimed in any preceding claim, wherein the first stage firing plateautemperature is in the range of 420° C. to 510° C.
 13. A method asclaimed in claim 12, wherein the first stage firing plateau temperatureis in the range of 480° C. to 490° C.
 14. A method as claimed in anypreceding claim, wherein the first stage firing duration is 15 mins to40 mins.
 15. A method as claimed in any preceding claim, wherein thefirst stage firing duration is 20 mins to 30 mins.
 16. A method asclaimed in any preceding claim, wherein the second stage firing plateautemperature is in the range of 630° C. to 710° C.
 17. A method asclaimed in claim 16, wherein the second stage firing plateau temperatureis 650° C. to 670° C.
 18. A method as claimed in any preceding claim,wherein the second stage firing duration is in the range of 5 mins to 35mins.
 19. A method as claimed in claim 18, wherein the second stagefiring duration is in the range of 5 mins to 15 mins.